Attempt to establish and to maintain in continuous culture hybrid cells for use in the treatment of spontaneously occurring and passively transferred murine leukemia. Leukemic mice will be treated with hybrid cells alone, or in combination with chemotherapy, radiotherapy or poly A:U. Efforts will be made to characterize immunogenic and nonimmunogenic hybrid cell lines with respect to expression of tumor associated antigens. Any modification of the study design must have the prior approval of the Project Officer before being instituted.